For decades scientists have been speculating about the origin of ultra-high energy cosmic rays, the incredibly powerful charged particles that travel across the cosmos, then set off a series of reactions as they smash through Earth's atmosphere. They're extremely rare, showing up only once per century for a given square mile.

Now astrophysicsts using the Pierre Auger Observatory in Argentina, which has detectors spread across 1,200 square miles, recently reported that they traced the rays back to their source. It appears likely that cosmic rays are born within active galactic nuclei. These galaxies, which have massive black holes at their centers, are busily churning up stars, gas and dust, generating hugely energetic reactions. The ultra-high energy rays are just one of the products. The work is published in the latest issue of the journal Science.—Gregory Mone